1. Field of the Invention
The present invention relates to an output circuit having an output terminal which assumes a logic output state generating a logic signal and a high impedance state (Hi-Z state) suitable for constructing a wired logic structure therewith.
2. Description of the Related Art
Open collector type or open drain type output circuits are widely utilized to form wired logic structures by connecting output terminals of the output circuits in common to a common data line. The open drain type output circuit has a structure in which a driving MOS transistor is connected between the output terminal and a ground voltage terminal and no load element is provided between the output terminal and a power voltage terminal within the output circuit. The wired-OR structure is realized by connecting in common output terminals of a plurality of open drain type output circuits to a common data line and an external load element such as a resistor element is provided between the common data line and the power voltage terminal. The common data line assumes a high level near the power voltage through the external load element when none of the driving transistors of the output circuits is energized and a low level approximately of the ground voltage when at least one of the driving transistor is energized. The common data line is provided with a relatively large capacitance which forms a time constant together with a resistance of the external load element. If the resistance of the external load element is set at a small value, it is possible to raise the potential at the common data line at a high speed when none of the driving transistors is energized. However, a large amount of DC current is caused through the external load element when at least one of the driving transistors is conductive, resulting in a large power consumption. In addition, in this instance the low level at the common data line is determined by a ratio of the resistance of the external load element and the on-resistance of the driving transistor, it is necessary to employ a large transistor as the driving transistor to attain the low level sufficiently close to the ground voltage. This lowers the integration density of the output circuit.